A flat-panel CRT display basically consists of an electron-emitting device and a light-emitting device that operate at low internal pressure. The electron-emitting device, commonly referred to as a cathode, contains electron-emissive elements that emit electrons over a wide area.
The emitted electrons are directed towards light-emissive elements distributed over a corresponding area in the light-emitting device. Upon being struck by the electrons, the light-emissive elements emit light that produces an image on the viewing surface of the display.
When the electron-emitting device operates according to field-emission principles, electrically conductive emitter material is commonly placed in series with the electron-emissive elements to gate the magnitude of current flow through the electron-emissive elements. FIG. 1 illustrates a conventional field-emission device, that so utilizes the conductive emitter material.
In the field emitter of FIG. 1, illustrates a multi-layer structure 100 which is a cross sectional view of a portion of an FED flat panel display. The multi-layer structure 100 comprises a field emission backplate structure 110. An image is generated at faceplate structure 120.
The backplate structure 110 generally comprises of an electrically insulating layer 111, a patterned gate electrode 112, and a conical electron-emissive element 113 situated in an aperture through insulating layer 111.
One type of electron emissive elements 113 is described in Spindt et al. U.S. Pat. No. 3,665,241. Spindt describes vertical field emission cathode/field ionizer structures in which “needle-like” elements such as conical or pyramidal tips are formed on a (typically conductive or semicondictive) substrate. Faceplate structure 120 is formed with a reflective layer 121, an anode comprising a reflective layer 121 and a black matrix layer 122, and a coating of phosphors 123.
The emission of electrons from electron-emissive elements 113 is controlled by applying a suitable voltage (Vg) to the gate 112. Another voltage (Ve) is applied directly to the electron emissive element 113 by way of the emitter electrode 114. Electron emission increases as the gate-to-emitter voltage, e.g., Vge is increased. Directing the electrons to the phosphor 123 is performed by applying a high voltage (Va) to the anode 120.
When a suitable gate-to-emitter voltage (Vge) is applied, electrons are emitted from electron-emissive element 113 at various values of off-normal emission angle theta 130. The emitted electrons follow non-linear trajectories indicated by lines 101 and impact on a target portion of phosphor 123. Thus, Vg and Ve determine the magnitude of the emission current (Ie) while the anode voltage (Va) controls the direction of the electron trajectories for a given electron emitted at a given time.
FIG. 2 illustrates another prior art electron-emissive structure. In the prior art structure shown in FIG. 2, a focus structure 115 is added to the backplate structure 110 to focus the electron beam more directly towards the faceplate structure 120. A focus voltage Vf is applied to the focus structure 115 to more accurate direct the electrons emitted from elements 113 to the phosphor 123.
FIG. 3 illustrates a portion of an exemplary FED screen 110. The FED screen 110 is subdivided into an array of horizontally aligned rows and vertically aligned columns of pixels. The boundaries of a respective pixel are indicated by dashed lines. Three separate row lines 311–313 are shown. Each row line 311–313 is a row electrode for one or the rows of pixel in the array.
In one embodiment, each row line 311–313 is coupled to the emitter electrodes of each emitter of the particular row associated with the electrode. A portion of one pixel row is shown in FIG. 2 and is situated between a pair of adjacent spacers. In color displays, each column of pixels has three column lines: (1) for red; (2) a second for green and (3) a third for blue. Likewise, each pixel column includes one of each phosphor stripes (red,green,blue) three stripes total.
In operation, the red, green and blue phosphor stripes are maintained at a high positive voltage relative to the voltage of the emitter electrode. When one of the sets of electron-emission elements is suitably excited by adjusting the voltage of the corresponding row lines and column lines, elements 113 in that set emit electrons which are accelerated towards a target portion of the phosphor in the corresponding color. The excited phosphors then emit light.
During a screen frame refresh cycle, each row line is activated in order from the first row to the last row and only one row is active at a given time and the column lines are energized to illuminate the one row of pixels for the on-time period. This is performed sequentially in time row-by-row.
According to an embodiment of the prior art, and anode is enabled by application of a predetermine threshold voltage (e.g. 300V). After the anode is enabled and has reached the threshold voltage, the emitter electrode and the gate electrode are the respectively enabled. The cathode may be enabled for a pre-determine period of time after the anode has been enabled to direct electrons towards the anode 120 and to prevent electrons from striking the gate electrode.
As a voltage differential is created by the gate electrode and the emitter electrode, electrons tend to strike the gate electrode rather than the anode 120. This situation can lead to overheating which may affect the voltage differential between the gate electrode and the emitters 113.
In addition, as the electrons jump the gap between the electron-emission elements 113 and the gate electrode, a luminous leakage of current may also be observed. Severe damage to the electron emitter may thus occur.
To solve the problem of electron depletion and discharge and the potential arching and depletion in the FED device.
In the device shown in FIG. 2, a focus structure comprising a focus electrode 115 is fabricated on the gate electrode 112 to provide a means of accurately focusing electrons to the anode structure 122. Although the focus structure 115 helps in alleviating the electron depletion problem of the prior art, this prior art solution adds additional fabrication steps to fabricating the FED device. This can be time consuming and costly.
It is therefore desirable to have a FED device that provides a reduce-electron depletion structure without additional fabricated structures in the device.